The present invention relates generally to the field of medical devices, and more particularly, to the stent grafts and their method of making.
Stents and similar endoluminal devices are currently used by medical practitioners to treat tubular body vessels or ducts that become so narrowed (stenosed) that flow of blood or other biological fluids is restricted. Such narrowing (stenosis) occurs, for example, as a result of the disease process known as arteriosclerosis. While stents are most often used to “prop open” blood vessels, they can also be used to reinforce collapsed or narrowed tubular structures in the respiratory system, the reproductive system, bile or liver ducts or any other tubular body structure. However, stents are generally mesh-like so that endothelial and other tissues can grow through the openings resulting in restenosis of the vessel.
Apart from use of stents within the circulatory system, stents have proven to be useful in dealing with various types of liver disease in which the main bile duct becomes scarred or otherwise blocked by neoplastic growths, etc. Such blockage prevents or retards flow of bile into the intestine and can result in serious liver damage. Because the liver is responsible for removing toxins from the blood stream, is the primary site for the breakdown of circulating blood cells and is also the source of vital blood clotting factors, blockage of the bile duct can lead to fatal complications. A popular type of stent for use in the biliary duct has been one formed from a shape memory alloy (e.g., nitinol) partially because such stents can be reduced to a very low profile and remain flexible for insertion through the sharp bend of the bile duct while being, self-expandable and capable of exerting a constant radial force to the duct wall.
Polytetrafluoroethylene (PTFE) has proven unusually advantageous as a material from which to fabricate blood vessel grafts or prostheses, tubular structures that can be used to replace damaged or diseased vessels. This is partially because PTFE is extremely biocompatible causing little or no immunogenic reaction when placed within the human body. This is also because in its preferred form, expanded PTFE (ePTFE), the material is light and porous and is readily colonized by living cells so that it becomes a permanent part of the body. The process of making ePTFE of vascular graft grade is well known to one of ordinary skill in the art. Suffice it to say that the critical step in this process is the expansion of PTFE into ePTFE. This expansion represents a controlled longitudinal stretching in which the PTFE is stretched to several hundred percent of its original length.
Cellular infiltration through stents can be prevented by enclosing the stents with ePTFE. Early attempts to produce a stent covered by ePTFE focused around use of adhesives or physical attachment such as suturing. However, such methods are far from ideal and suturing, in particular, is very labor intensive. More recently methods have been developed for encapsulating a stent between two tubular ePTFE members whereby the ePTFE of one-member touches and bonds with the ePTFE of the other member through the mesh opening in the stent. However, such a monolithically encapsulated stent may tend to be rather inflexible. Moreover, even covered stents that include slit cut and bridge connection designed graft coverings tend to be inflexible because the covering graft material is unable to expand lengthwise with the underlying stent frame.
Other solutions to provide a more flexible stent graft include a stent graft device described in U.S. Pat. No. 6,579,314 which is incorporated herein in its entirety by reference thereto. U.S. Pat. No. 6,579,314 describes a flexible stent graft that uses a partially encapsulated stent having areas covered by only a single layer of ePTFE in order to provide flexibility to the stent graft device. Another partially encapsulated stent is shown and described in U.S. Pat. No. 6,558,414 which is also incorporated herein in its entirety by reference thereto.
Other solutions provide for making a self-expanding stent longitudinally expandable. For example, U.S. Pat. No. 5,899,935 includes a method of manufacturing a stent in which the stent is stretched longitudinally to reduce its outer diameter and coated in a material to freeze the stretched configuration. In the description of use, the coating is disintegrated to permit the stent to expand.